Method and device for determining the position and the rotational speed of a rotor of an electric machine

ABSTRACT

The invention relates to a method for determining the position (OR) and the rotational speed (nR) of a rotor of an electrical machine during an active short circuit and a rotor-state determining device (10) designed to carry out the method. The method comprises the steps of determining the short circuit currents (Iu, Iv, Iw) resulting during the short circuit, determining a total current (Iα, Iβ) resulting from the short circuit currents (Iu, Iv, Iw), determining a stator current angle (ψl) of the total current (Iα, Iβ) with respect to a stator coordinate system (α, β), determining a rotor current angle (φl) of the total current (Iα, Iβ) with respect to a flux direction (dR) of the rotor, this step comprising the steps of calculating an amount variable (I) of the total current (Iα, Iβ), determining the rotor current angle (φl) on the basis of a characteristic dependence between the amount variable (I) and a rotor current angle (φl), which dependence is created for the electrical machine, the rotor position (θR) corresponding to a sum of the stator current angle (ψl) and the rotor current angle (φl), and the rotor rotational speed (nR) resulting from monitoring of the rotor position (θR).

BACKGROUND OF THE INVENTION

The present invention relates to a method for determining a position androtational speed of a rotor of an electric machine during an activeshort circuit.

Electric machines such as, for example, permanently excited synchronousmachines, are used in numerous technical areas. For example, suchpermanently excited synchronous machines are used in motor vehicles, inparticular electric vehicles and hybrid vehicles.

WO 2016/066330 discloses a method and a device for switching from theopen-circuit operating state of an electric machine into a short-circuitoperating state.

The background of the invention is that in the event of a failure of therotor position sensors at a rotational speed of the rotor within anemergency operation rotational speed range, the electric machineswitches into the emergency operating mode. In order to avoid damage tothe electric machine, automatic switching into an active short circuitoccurs above an emergency operation rotational speed. However, thismakes it impossible to operate the electric machine in an emergencyoperating mode.

SUMMARY OF THE INVENTION

Therefore the object of the present invention is to make available amethod and a device with which in the event of a failure of rotorposition sensors, the position and the rotational speed of the rotor ofan electric machine can be determined during an active short circuit sothat a safe transition into the emergency operating mode is possible.

The object is achieved by means of a method for determining a positionand rotational speed of a rotor of an electric machine during an activeshort circuit.

The method according to the invention comprises the steps of determiningthe short circuit currents occurring during the short circuit,determining a total current arising from the short circuit currents,determining a stator current angle of the total current with respect toa stator coordinate system, determining a rotor current angle of thetotal current with respect to a direction of flow of the rotor. Thelatter comprises here the steps of calculating an absolute-valuevariable of the total current, determining the rotor current angle onthe basis of a characteristic dependence, produced for the electricmachine, between the absolute-value total current and a rotor currentangle, wherein the rotor position corresponds to a sum of the statorcurrent angle and the rotor current angle, and wherein the rotorrotational speed is obtained from monitoring the rotor position.

The position of the rotor according to the present invention isunderstood to be the position of the rotor relative to a stator. Thisposition is preferably indicated here as an angle. Accordingly, therotational speed of the rotor is also to be understood to be a relativemovement of the rotor with respect to the stator. The stator currentangle or rotor current angle is understood here to be an angle betweenthe stator or rotor and a total current.

The advantage of the invention is that even in the event of failure ofthe rotor position sensors the position and the rotor rotational speedcan be determined. As a result, during the active short circuit theelectric machine can be safely changed into an emergency operating mode.When an emergency operation rotational speed is reached, the emergencyoperating mode can therefore be started without damage occurring to theelectric machine.

In one preferred embodiment of the invention, the characteristicdependence is determined by relating to one another therotational-speed-dependent value of absolute-value total current and ofthe rotor current angle. This makes it possible to calculate a rotorcurrent angle by determining an absolute-value total current.

In a further preferred embodiment of the invention, a rotor positionsmoothed value is determined from smoothing a multiplicity of rotorposition values. Consequently, inaccuracies as a result of largedeviations in individual values can be minimized. Measured value noiseis as a result reduced so that the accuracy of the determined rotorposition is increased.

The method preferably comprises a step in which the determined rotorposition and/or rotor rotational speed are/is compared with the sensorinformation. This can permit a defect of a sensor to be verified oncemore.

The invention additionally comprises a rotor state-determining devicewhich is configured to carry out the method according to the invention.The rotor state-determining device comprises here a current-determiningunit for determining the short circuit currents which occur during theshort circuit, a total current-determining unit for determining a totalcurrent which results from the short circuit currents, a stator currentangle-determining unit for determining a stator current angle of thetotal current with respect to a stator coordinate system, a rotorcurrent angle-determining device for determining a rotor current angleof the total current with respect to a direction of flow of the rotor,wherein the rotor current angle-determining device comprises acalculation unit for calculating an absolute-value variable of the totalcurrent, and a current angle-correlation unit for determining the rotorcurrent angle which is correlated with the absolute-value total current,a rotor position-determining unit for determining a rotor position, arotor rotational speed-determining device for determining a rotorrotational speed.

The method according to the invention can be carried out by means of therotor state-determining device so that the advantages specified withrespect to this method can be achieved.

In one preferred embodiment, the rotor rotational speed-determiningdevice comprises a smoothing unit. The method for smoothing the rotorposition values can be carried out by means of this smoothing unit, sothat the accuracy of the determined rotor position values is increased.

In a further preferred embodiment, the electric machine is a permanentmagnetic synchronous machine, an electrically excited synchronousmachine or a synchronous reluctance machine. The permanent magnetsynchronous machine has the advantage that it has a high level ofefficiency and a small design. The synchronous reluctance machine hasthe advantage that in contrast with permanent magnet-excited synchronousmotors no magnetic materials based on so-called rare earth materials areused in its manufacture. Also, virtually no losses occur in the rotor ofa synchronous reluctance machine, and as result it has a good level ofefficiency.

The electric machine preferably has three phases. The advantage of suchelectric machines is that they have high dynamics, a high torque andhigh efficiency.

In order to carry out the method according to the invention, theinvention comprises a computer program product having program code meansfor carrying out the method when the computer program product is storedon a control unit of a rotor-state-determining device or on acomputer-readable data carrier. Moreover, the invention comprises amachine-readable storage medium in which the computer program product isstored.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are illustrated in the drawingand explained in more detail in the following description. In thedrawing:

FIG. 1 shows an exemplary embodiment of a method and of a rotorstate-determining device for determining a position and rotational speedof a rotor of an electric machine,

FIG. 2 shows a graphic illustration of a position determination processof a rotor with respect to a stator using the example of a three-phaseelectric machine,

FIG. 3 shows diagrams relating to the derivation of a characteristicdependence between the absolute-value total current and a rotor currentangle, and

FIG. 4 shows a diagram of the characteristic dependence between theabsolute-value total current and a rotor current angle.

DETAILED DESCRIPTION

FIG. 1 shows an exemplary embodiment of a method and an exemplaryembodiment of a rotor state-determining device 10 for determining aposition OR and rotational speed n_(R) of a rotor (not shown) of anelectric machine (not shown). For this purpose, the short circuitcurrents I_(U), I_(V), I_(W) occurring during an active short circuitare determined in a current-determining unit 14, using the example of athree-phase electric machine. The resulting short circuit currentsI_(U), I_(V), I_(W) are illustrated in FIG. 2. A total current I_(α),I_(β) arising from the short circuit currents I_(U), I_(V), I_(W) isthen determined in a total current-determining unit 18 of the rotorstate-determining device 10. A graphic determination of the totalcurrent I_(α), I_(β) from the short circuit currents I_(U), I_(V), I_(W)which are offset by 120° for a three-phase motor is illustrated in FIG.2.

A stator current angle-determining unit 22 determines an angle, denotedas a stator current angle ψ_(I) and shown in FIG. 2, of the totalcurrent I_(α), I_(β) with respect to a stator coordinate system α, β. Adirection of flow of the total current I_(α), I_(β) with respect to thestator coordinate system α, β is indicated by means of the statorcurrent angle ψ_(I). The rotor state-determining device 10 additionallycomprises a rotor current angle-determining device 26 which comprises acalculation unit 30 in which an absolute-value variable I of the totalcurrent I_(α), I_(β) is calculated.

Moreover, the rotor current angle-determining device 26 comprises acurrent angle-correlation unit 34 which determines a rotor current angle(pi on the basis of a characteristic dependence, produced for theelectric machine, between the absolute-value total current I determinedby the calculation unit 30 and a rotor current angle φ_(I).

As shown in FIG. 2, the rotor current angle φ_(I) indicates the anglebetween the total current I_(α), I_(β) and the direction of flow d_(R)of the rotor. The rotor state-determining device 10 additionallycomprises a rotor position-determining unit 38 which calculates therotor position Θ_(R) from a sum of the stator current angle ψ_(I) androtor current angle φ_(I). The rotor position OR corresponds, as shownin FIG. 2, to the angle between the direction of flow d_(R) of the rotorwith respect to the stator. The rotor rotational speed n_(R) isdetermined by means of a rotor rotational speed-determining device 42,by monitoring the rotor position Θ_(R).

A rotor position smoothed value Θ_(Rg) and rotor rotational speedsmoothed value n_(Rg) are calculated from a multiplicity of rotorposition values Θ_(R) and rotor rotational speed values n_(R) in asmoothing unit 46. Consequently, inaccuracies as a result of largedeviations in individual values can be minimized.

FIG. 3 shows diagrams relating to the derivation of a characteristicdependence between the absolute-value total current I and a rotorcurrent angle φ_(I). For this purpose, the absolute-value total currentI determined from the total current I_(α), I_(β) and the rotor currentangle (pi have been respectively determined for an electric machine as afunction of the machine rotational speed n. Theserotational-speed-dependent values which are determined in the processare different for each electric machine and characterize said machine.

The absolute-value total current I and the rotor current angle (pi canbe related to one another by means of the same machine rotational speedvalues n without a rotational speed being known. Such a relationshipbetween the two values is shown in FIG. 4. In this figure, thecharacteristic dependence between the absolute-value total current I andthe rotor current angle φ_(I) is shown. By means of this diagram, thecurrent angle-correlation unit 34 determines, on the basis of theabsolute-value total current I, the rotor current angle φ_(I) associatedtherewith.

1. A computer-implemented method for determining a position (Θ_(R)) androtational speed (n_(R)) of a rotor of an electric machine during anactive short circuit, wherein the method comprises: determining theshort circuit currents (I_(U), I_(V), I_(W)) occurring during the shortcircuit, determining a total current (I_(α), I_(β)) arising from theshort circuit currents (I_(U), i_(V), I_(W)), determining a statorcurrent angle (ψ_(I)) of the total current (I_(α), I_(β)) with respectto a stator coordinate system (α, β, determining a rotor current angle(φ_(I)) of the total current (I_(α), I_(β)) with respect to a directionof flow (d_(R)) of the rotor by: calculating an absolute-value variable(I) of the total current (I_(α), I_(β)), determining the rotor currentangle NO on the basis of a characteristic dependence, produced for theelectric machine, between the absolute-value total current (I) and arotor current angle (φ_(I)), wherein the rotor position (Θ_(R))corresponds to a sum of the stator current angle (ψ_(I)) and the rotorcurrent angle (φ_(I)), and wherein the rotor rotational speed (n_(R)) isobtained from monitoring the rotor position (Θ_(R)).
 2. The method fordetermining a position (Θ_(R)) and rotational speed (n_(R)) of a rotorof an electric machine as claimed in claim 1, wherein the characteristicdependence is determined by relating to one another therotational-speed-dependent value of absolute-value total current (I) andof the rotor current angle (φ_(I)).
 3. The method for determining aposition (Θ_(R)) and rotational speed (n_(R)) of a rotor of an electricmachine as claimed in claim 1, wherein a rotor position smoothed value(Θ_(Rg)) is determined from smoothing a multiplicity of rotor positionvalues (Θ_(R)).
 4. The method for determining a position (Θ_(R)) androtational speed (n_(R)) of a rotor of an electric machine as claimed inclaim 1, wherein the method comprises a step in which the determinedrotor position (Θ_(R)) and/or rotor rotational speed (n_(R)) are/iscompared with the sensor information.
 5. A rotor state-determiningdevice (10) comprising: a current-determining unit (14) for determiningthe short circuit currents (I_(U), I_(V), I_(W)) which occur during theshort circuit, a total current-determining unit (18) for determining atotal current (I_(α), I_(β)) which results from the short circuitcurrents (I_(U), I_(V), I_(W)), a stator current angle-determining unit(22) for determining a stator current angle (ψ_(I)) of the total current(I_(α), I_(β)) with respect to a stator coordinate system (α, β), arotor current angle-determining device (26) for determining a rotorcurrent angle (ψ_(I)) of the total current (I_(α), I_(β)) with respectto a direction of flow (d_(R)) of the rotor, wherein the rotor currentangle-determining device (26) comprises a calculation unit (30) forcalculating an absolute-value variable (I) of the total current (I_(α),I_(β)), and a current angle-correlation unit (34) for determining therotor current angle (ψ_(I)) which is correlated with the absolute-valuetotal current (I), a rotor position-determining unit (38) fordetermining a rotor position (Θ_(R)), a rotor rotationalspeed-determining device (42) for determining a rotor rotational speed(n_(R)).
 6. The rotor state-determining device (10) as claimed in claim5, wherein the rotor rotational speed-determining device (42) comprisesa smoothing unit (46).
 7. The rotor state-determining device (10) asclaimed in claim 5, wherein the electric machine is a permanent magneticsynchronous machine, an electrically excited synchronous machine or asynchronous reluctance machine.
 8. The rotor state-determining device(10) as claimed in claim 5, wherein the electric machine has threephases.
 9. A non-transitory, computer-readable medium containinginstructions that when executed by by a computer in a control unit (14,18, 22, 26, 38, 42) of a rotor state-determining device (10) cause thecomputer to determine the short circuit currents (I_(U), I_(V), I_(W))occurring during the short circuit, determine a total current (I_(α),I_(β)) arising from the short circuit currents (I_(U), I_(V), I_(W)),determine a stator current angle (ψ_(I)) of the total current (I_(α),I_(β)) with respect to a stator coordinate system (α, β), determine arotor current angle (ψ_(I)) of the total current (I_(α), I_(β)) withrespect to a direction of flow (d_(R)) of the rotor by: calculate anabsolute-value variable (I) of the total current (I_(α), I_(β)),determine the rotor current angle (φ_(I)) on the basis of acharacteristic dependence, produced for the electric machine, betweenthe absolute-value total current (I) and a rotor current angle (φ_(I)),wherein the rotor position (Θ_(R)) corresponds to a sum of the statorcurrent angle (ψ_(I)) and the rotor current angle (φ_(i)), and whereinthe rotor rotational speed (n_(R)) is obtained from monitoring the rotorposition (Θ_(R)).
 10. (canceled)